Additional Money
This requires you to do some file editing, but it's very easy. Open the zoo.ini in your Zoo Tycoon directory with notepad or a similar text editor. Find the line that says "MSMaxCash". Change the figure next to it to whatever you want. This will change the maximum amount of money you can choose from when you start a zoo in the sandbox mode, for example

Developer Mode
If you have one of the BETA versions, hit Ctrl-D to access the Developer Mode. It will allow you to move zoo entrances and fences, add odd buildings, and even add vehicles. Remember, this only works in the BETA builds of Zoo Tycoon. I tried it with the full version, and it didn't work.

Make Trees or Grid Disappear/Appear
By pressing CRTL + F, you can make all trees in your zoo ''disappear'' (they still exist, they are just invisible for building purposes). Select CRTL + F again to make them reappear

By pressing CRTL + G, you can make the main zoo grid appear of dissappear, allowed you to judge the zoo from a regular smooth vantage point, or see each individual square.

Unlock Triceratops
Name one of your exhibits "Cretaceous Corral". This will unlock the Triceratops. I should also note that if you name a brand new exhibit using this cheat, it will crash the game. Everyone who's done it with a new exhibit has experienced this, so keep it in mind.

Yellow brick road
Make an exhibit with a lion, a tiger and a bear. This will unlock the yellow brick path.

Exhibit codes
Name one of your exhibits accordingly for the desired effect. It should also be noted that if you name a brand new exhibit using this cheat, it will crash the game. Everyone who's done it with a new exhibit has experienced this, so keep it in mind.
Microsoft = Double Exhibit Donations
Blue Fang = Guests pay 2x original amount for attractions
Wonderland = More people come
Xanadu = unlock the Unicorn

Note: if you name a brand new exhibit using this cheat, it will crash the game. Everyone who's done it with a new exhibit has experienced this, so keep it in mind.